Various analytical instruments can be used for analyzing proteins and other biomolecules. More recently, mass spectrometry has gained prominence because of its ability to handle a wide variety of biomolecules with high sensitivity and rapid throughput. A variety of ion sources have been developed for use in mass spectrometry. Many of these ion sources comprise some type of mechanism that produces charged species through spraying. One particular type of technique that is often used is Electrospray Ionization (“ESI”). One benefit of ESI is its ability to produce charged species from a wide variety of biomolecules such as proteins. Another benefit of ESI is that it can be readily used in conjunction with a wide variety of chemical separation techniques, such as High Performance Liquid Chromatography (“HPLC”). For example, ESI is often used in conjunction with HPLC for identifying proteins.
Typically, ESI produces a spray of ions in a gaseous phase from a sample stream that is initially in a liquid phase. For a conventional ESI mass spectrometry system, a sample stream is pumped through a metal capillary, while a relatively high electric field is applied between a tip of the metal capillary and an electrode that is positioned adjacent to the tip of the metal capillary. As the sample stream exits the tip of the metal capillary, surface charges are produced in the sample stream, thus pulling the sample stream towards the electrode. As the sample stream enters the high electric field, a combined electro-hydrodynamic force on the sample stream is balanced by its surface tension, thus producing a “Taylor cone.” Typically, the Taylor cone has a base positioned near the tip of the metal capillary and extends up to a certain distance away from the tip of the metal capillary, beyond which a spray of droplets is produced. As these droplets move towards the electrode, coulombic repulsive forces and desolvation lead to the formation of a spray of ions in a gaseous phase.
During operation of a conventional ESI mass spectrometry system, characteristics of a Taylor cone can affect characteristics of a spray of ions, which, in turn, can affect results of mass spectrometric analysis. Accordingly, it is desirable to produce Taylor cones with certain reproducible characteristics, such that results of mass spectrometric analysis have a desired level of accuracy and reproducibility.